PROJECT SUIVIMARY Some form of myelosuppression is necessary for effective and durable engraftment of transplanted genetically corrected autologous stem cells. Milder conditioning protocols which support efficient engraftment but do not subject patients to undue toxicity and risks are highly desirable, but these have not been tested in large animals or humans as yet. In this project we propose too investigate two such innovative conditioning regimens in murine and primate preclinical models. The proposed experiments test the hypothesis that low dose irradiation in combination with either Ab-mediated impairment of stem cell function or with the use of stem cell mobilizing agents will facilitate the engraftment of genetically corrected stem cells. Thus, low dose irradiation will be combined in Specific Aim 1 with anti-human c-kit treatment to reduce the stem/cell progenitor pool and in Specific Aim 2 with a mobilizing agent for displacement of stem/progenitor cells. Ancillary preliminary experiments using similar regimens will be conducted in mice to provide guidance and mechanistic clues. Since not only recipient conditioning is necessary for enhanced engraftment, innovative treatments of donor cells are also important. We will address this issue in Specific Aim 3. To enhance engraftment, in this Specific Aim we exploit short term treatment of in vitro incubated cells of murine or human origin with dmPGE2 and their transplantation in an immunodeticient host (NOD/SCID). Comparison of the behavior of murine vs. human cells treated ex wVo with dmPGE2 will be done in the same immunodeficient recipient model allowing also the approach of mechanistic issues. We believe that results from these experiments will have important clinical implications in transplantation outcomes in humans using autologous gene corrected cells.